1. Technical Field
The present disclosure relates to electrosurgery. More particularly, the present disclosure relates to electrosurgical systems and methods for compensating for the impedance of a cable used to deliver electrosurgical energy to tissue when the electrosurgical energy has energy at frequencies other than a fundamental frequency.
2. Background of Related Art
Electrosurgery involves the application of high-frequency electric current to cut or modify biological tissue during an electrosurgical operation. Electrosurgery is performed using an electrosurgical generator, an active electrode, and a return electrode. The electrosurgical generator (also referred to as a power supply or waveform generator) generates an alternating current (AC), which is applied to a patient's tissue through the active electrode and is returned to the electrosurgical generator through the return electrode. The alternating current typically has a frequency above 100 kilohertz (kHz) to avoid muscle and/or nerve stimulation.
During electrosurgery, the AC generated by the electrosurgical generator is conducted through tissue disposed between the active and return electrodes. The electrical energy (also referred to as electrosurgical energy) delivered to the tissue is converted into heat due to the resistivity of the tissue, which causes the tissue temperature to rise. The electrosurgical generator controls the heating of the tissue by controlling the electric power (i.e., electrical energy per time) provided to the tissue. Although many other variables affect the total heating of the tissue, increased current density and resistance of the tissue usually lead to increased heating. The electrosurgical energy is typically used for cutting, dissecting, ablating, coagulating, and/or sealing tissue.
The two basic types of electrosurgery employed are monopolar and bipolar electrosurgery. Both of these types of electrosurgery use an active electrode and a return electrode. In bipolar electrosurgery, the surgical instrument includes an active electrode and a return electrode on the same instrument or in very close proximity to one another, which cause current to flow through a small amount of tissue. In monopolar electrosurgery, the return electrode is located elsewhere on the patient's body and is typically not a part of the electrosurgical instrument itself. In monopolar electrosurgery, the return electrode is part of a device typically referred to as a return pad.
Electrosurgical generators make use of voltage and current sensors to measure quantities, such as power and tissue impedance, for controlling the output of the electrosurgical generator to achieve a desired clinical effect. The voltage and current sensors are often located inside the electrosurgical generators to save costs associated with incorporating sensors into the surgical instruments.
A cable, which may be more than a meter in length, connects the electrosurgical generator to the active and return electrodes and is used to deliver electrosurgical energy to tissue being treated. Every cable has an impedance that includes an inductance, a capacitance, and a resistance. This impedance can change the amount of actual energy delivered to the tissue in two ways. For a low load impedance, the inductance and resistance of the cable reduces the amount of voltage delivered to the tissue proportional to the amount of current, that is, as the current increases, the voltage drop across the cable will also increase. For a high load impedance, as the voltage increases, the amount of current flowing through the capacitance of the cable increases. This reduces the amount of current that is delivered to the tissue and adds distortions to the voltage and current waveforms so that they deviate from the desired pure sinusoidal, rectangular, sawtooth, pulse, triangular, or blended waveforms commonly used for electrosurgery.
Additionally, certain types of waveforms, such as pulse waveforms, have a significant amount of energy at frequencies other than the fundamental frequency. Thus, to more accurately measure power and impedance, many generators employ compensation algorithms that account for the cable impedance. These compensation algorithms typically involve solving Kirchhoff current and voltage equations for multiple nodes in a circuit model that models the impedance in the generator and cable as a circuit network. However, solutions to these equations, when implemented by a real-time embedded software system, may require a significant amount of memory and processing power.